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a tremendous array of biologically important compounds in all living things. Some of these compounds are released as waste, or eventually the organism itself dies. They are then broken down, mainly by microorganisms, and eventually the carbon returned to inorganic form.

With carbon's central importance to all known forms of life, perhaps it is not surprising that the redox reactions through which these atoms are cycled (Figure 13.3) have such a high degree of breadth, complexity, and variability. In fact, its role as the dominant element in living organisms appears to stem from the great diversity of its bonding options. It typically forms four bonds with adjacent atoms. This is more than hydrogen (one bond) or oxygen (two), but not really different from nitrogen (three or four), and less than phosphorus (five) or sulfur (two to six). However, its great versatility also

Carbon Oxidation States

Carbon Oxidation States

stems from the fact that it can occur in all of the oxidation states from —4 to +4 (Table 13.2).

Organic Carbon What exactly is meant by organic carbon? The term dates to the time that such compounds were considered to be produced only by organisms. German Frei-drich Wohler is generally credited with the first synthesis of an organic compound, urea, in 1928. Now, of course, there are many synthetic organic compounds. So again, what does organic mean?

Interestingly, most chemistry texts do not provide a real definition. A dictionary may state that organic chemistry is the study of carbon compounds, but carbon dioxide is not considered organic (although it is produced by organisms). Another definition that has been used is "compounds containing a C—C bond.'' This would exclude carbon dioxide, as desired, but would also exclude methanol, formic acid, and formaldehyde, which are generally considered organic. (Formic acid, for example, is produced by ants for their sting, as noted in the scientific name of the family, Formicidae.) Another definition is "compounds containing a C—H bond.'' This would exclude carbon dioxide and include methanol, formic acid, and formaldehyde. It would also include methane, which some people consider organic, but not carbon tetrachloride. Interestingly, neither definition would include urea!

Perhaps this explains why most texts avoid the question—there is not a simple answer. Once again, nature does not easily fit into human classification systems. For our purposes, we will consider carbon-containing compounds (not the elemental forms graphite and diamond) to be organics, with the exception of carbon dioxide, carbonic acid, bicarbonate, carbonate, and carbon monoxide. Methane we consider both organic, since it is the carbon and energy source for methanotrophs (Section 10.5.6), and inorganic, since it is a major end product of anaerobic mineralization and also the product of carbon dioxide respiration (Section 13.1.1).

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